Issue Resolution To make sure non-Marketo landing page URL will show up in Smart List's filters, the following steps have to be done.

Make sure the Marketo Munchkin Tracking code is implemented and placed within the non-Marketo webpages.

The non-Marketo URLs would show in the filter if a known lead/person has visited the web page and tracked into the activity log. Therefore, make a lead/person and visit the webpage to allow Marketo to track and log that URL visit.